LTC3805-5 Linear Technology, LTC3805-5 Datasheet - Page 14

• Manufacturer Part Number: LTC3805-5
• Description: Adjustable Frequency Current Mode Flyback DC/DC Controller
• Manufacturer: Linear Technology

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LTC3805-5
APPLICATIONS INFORMATION
14
For example, with the peak current sense voltage of 100mV
on the I
a sense resistor of 0.020Ω. Note that the instantaneous
peak power in the sense resistor is 0.5W and it must be
rated accordingly. The LTC3805-5 has only a single sense
line to this resistor. Therefore, any parasitic resistance
in the ground side connection of the sense resistor will
increase its apparent value. In the case of a 0.020Ω sense
resistor, one milliohm of parasitic resistance will cause a
5% reduction in peak switch current. So the resistance of
printed circuit copper traces and vias cannot necessarily
be ignored.
Programmable Slope Compensation
The LTC3805-5 injects a ramping current through its I
pin into an external slope compensation resistor R
This current ramp starts at zero right after the GATE pin
has been high for the LTC3805-5’s minimum duty cycle
of 6%. The current rises linearly towards a peak of 10μA
at the maximum duty cycle of 80%, shutting off once the
GATE pin goes low. A series resistor R
I
ramping voltage drop. From the perspective of the I
pin, this ramping voltage adds to the voltage across the
sense resistor, effectively reducing the current comparator
threshold in proportion to duty cycle. This stabilizes the
SENSE
pin to the current sense resistor R
SENSE
Figure 4. Circuit for Isolated Feedback
pin, a peak switch current of 5A requires
LTC3805-5
GND
V
I
CC
TH
FB
38055 F04
ISOLATION
BARRIER
SLOPE
SENSE
connecting the
develops a
SLOPE
SENSE
SENSE
.
control loop against subharmonic oscillation. The amount
of reduction in the current comparator threshold (ΔV
can be calculated using the following equation:
Note: LTC3805-5 enforces 6% < Duty Cycle < 80%. A good
starting value for R
in current comparator threshold at 80% duty cycle.
Designs that do not operate at greater than 50% duty cycle
do not need slope compensation and may replace R
with a direct connection.
Overcurrent Threshold Adjustment
Figure 5 shows the connection of the overcurrent pin OC
along with the I
R
which is driven by the GATE pin. The internal overcurrent
threshold on the OC pin is set at V
same as the peak current sense voltage V
the I
ment resistor R
I
the overcurrent threshold adjust current I
external resistor R
SLOPE
SENSE
ΔV
SENSE
Figure 5. Circuit to Decrease Overcurrent Threshold
SENSE
is discussed in the prior section. In combination with
located in the source circuit of the power NMOS
pin. The role of the slope compensation adjust-
=
LTC3805-5
SLOPE
DutyCycle
SENSE
I
SENSE
GATE
OC
GND
SLOPE
OC
can be used to lower the overcurrent
and the slope compensation current
80
pin and the current sense resistor
R
%
SLOPE
R
is 3k, which gives a 30mV drop
OC
− 6
I
%
OC
I
OCT
SLOPE
= 10μA
10
38055 F05
= 100 mV which is the
μ
A R
I(MAX)
R
•
SENSE
OC
SLOPE
= 100 mV on
= 10μA, an
SENSE
SLOPE
38055fa
)